The general test for detection of carbohydrates

Dehydration When treated with concentrated sulfuric acid, monosaccharides undergo dehydration with an elimination of 3 water molecules. Thus hexoses give hydroxymethyl furfural while pentoses give furfural on dehydration. These furfural s can condense with phenolic compounds (a-naphthol) to form coloured products. This is the chemical basis of the popular Molisch test. In case of oligo- and polysaccharides, they are first hydrolysed to monosaccharides by acid, and this is followed by dehydration. The test solution is combined with a small amount of Molisch's reagent (a-naphthol dissolved in ethanol) in a test tube. After mixing, a small amount of concentrated sulfuric acid is slowly added down the sides of the sloping test-tube, without mixing, to form a layer. A positive reaction is indicated by appearance of a purple red ring at the interface between the acid and test layers Ref : Biochemistry by U. Satyanarayana 3rd edition Pgno : 17